In the glass container industry, small cracks or fracture in the glass are referred to as “check defects”. Checks can range from sub millimeters to several hundred millimeters and can be oriented at any direction from vertical to horizontal. Glass is not a crystalline structure by nature, but most cracks propagate roughly along a plane of some orientation in space mostly determined by the shape of the glass at that location. For example, a crack that began as a vertical crack at the upper surface of the mouth primarily propagates in a vertical plane. Checks can appear in any orientation and on any portion of a container and can exist wholly within the glass or may penetrate to one or both surfaces. Checks are considered phase objects and do not absorb light like a solid objects does. Checks are primarily reflective in nature if their opposed surface separation is at least half a wavelength of light. However, very few checks with a smaller separation will reflect light and accordingly they will not likely be detectable by direct reflection methods, but they might have scattering points when they penetrate to the one or both surfaces of the container and will scatter light back to the sensors.
Most of these crack defects will drastically weaken the bottle, often causing it to rupture or to leak. Therefore, bottle manufacturers like to remove these containers before they reach filling plants. Checks appearing near the mouth of the containers are called finish checks. In the glass bottle industry, the term “container finish” refers to the portion of the bottle that defines the mouth, threads or beads, and the ring. The upper surface of the mouth is referred as the sealing surface.
Almost all commercially available check detectors work on the principle of reflected light. A conventional check detector consists of a series of continuously operating light spot light sources and associated photodetectors that are positioned so that known checks on a bottle rotating at an inspection station will reflect light from one of the sources to one of the photo-detectors. Signal processing of the photodetector outputs recovers the sharp peaks while rejecting lower frequency signal variations caused by ambient light, reflection from the bottle sidewall, etc.
While commercially available check detectors are successfully deployed on most glass bottle production lines, there are several drawbacks to the approach. A few of those are: many point sensors are required for many possible reflection angles; some sensor angles are difficult to position; additional sensors and lights need to be added as more production defects appear; time consuming setup is required for each type of container; and it is difficult to reproduce the same setup from one inspection line to another.
The following U.S. Pat. Nos. 4,701,612, 4,945,228, 4,958,223, 5,020,908, 5,200,801, 5,895,911, 6,104,482, 6,211,952, and 6,275,287 all relate to devices that detect defects in the finish of a container.